Investigation of cation environment and framework changes in silicotitanate exchange materials using solid-state Na, Si, and Cs MAS NMR

Crystalline silicotitanate (CST), HNa₃Ti₄Si₂O₁₄·4H₂O and the Nb-substituted CST (Nb-CST), HNa₂Ti₃NbSi₂O₁₄·4H₂O, are highly selective Cs⁺ sorbents, which makes them attractive materials for the selective removal of radioactive species from nuclear waste solutions. The structural basis for the improved Cs⁺ selectivity in the niobium analogs was investigated through a series of solid-state magic angle spinning (MAS) NMR experiments. Changes in the local environment of the Na⁺ and Cs⁺ cations in both CST and Nb-CST materials as a function of weight percent cesium exchange were investigated using ²³Na and ¹³³Cs MAS NMR. Framework changes induced by Cs⁺ loading and hydration state were investigated with ²⁹Si MAS NMR. Multiple Cs⁺ environments were observed in the CST and Nb-CST material. The relative population of these different Cs⁺ environments varies with the extent of Cs⁺ loading. Marked changes in the framework Si environment were noted with the initial incorporation of Cs⁺, however with increased Cs⁺ loading the impact to the Si environment becomes less pronounced. The Cs⁺ environment and Si framework structure were influenced by the Nb-substitution and were greatly affected by the amount of water present in the materials. The increased Cs⁺ selectivity of the Nb-CST materials arises from both the chemistry and geometry of the tunnels and pores.     

Formation of Ga2Te2 and M3Te3(M = Ga, In) rings from reactions of sodium ditelluroimidodiphosphinate with Group 13 halides

Metathetical reactions of Na[N(iPr2PTe)2] with Group 13 metal halides produce the telluride complexes {Ga(mu-Te)[iPr2PNiPr2PTe]}2(2) and {M(mu-Te)[N(iPr2PTe)2]}3, M = In (3) and Ga (4), which contain central Ga2Te2 and M3Te3 rings, respectively.     

Analyse systematique des elements traces dans l'eau de mer par activation neutronique

The trace elements in sea water, particularly the pollutant elements (Cu, Cd, As, Zn, Hg and Cr) are present in very low concentrations (in the order of 10^?9 g/ml). For the determination of the concentration of these elements the neutron activation analysis has been used successfully. The separation of the impurities has been studied using radioactive tracers by coprecipitation with carriers and on ion exchange resins. The preconcentration technics applied before the irradiation introduces too large “blank” values, which can be reduced by etching the containers with HF—EDTA. In order to have representative samples, the water must be quickly frozen after taking out from the sea. With these precautions, the method developed allows to analyse samples of 3 ml volumes. The results of sea water sample analyses from different areas are presented.     

Speciation studies for arsenic and mercury-applications and prospects

Methods are described for the speciation of arsenic and mercury in biological and environmental materials. For arsenic various methods were developed to distinguish between more or less toxic arsenic compounds and non-toxic compounds. For the determination of methylmercury a modification of the Westöö procedure was applied for higher contents as well as anion exchange down to levels below 0.1 g/kg in solids and below 0.1 ng/1 in liquids.     

Influence of Higher Excited Singlet States on Ultrafast Exciton Motion in Pigment-Protein Complexes

Abstract— Numerical simulations of the ultrafast exciton motion in photosynthetic antenna complexes are used to reproduce measured data of optical pump-probe experiments. Emphasis is put on a chlorophyll aL/chlorophyll b dimer of the light-harvesting complex of the photosystem II of higher plants (LHC-II). To account for intramolecular excited-state absorption the standard exciton theory is extended to the inclusion of a second higher excited singlet state per chlorophyll molecule. The density matrix theory is applied to describe the dissipative dynamics of excitons. Different mechanisms for energy relaxation and dephasing including pure dephasing processes are discussed. As a result, a further refinement of earlier calculations on the one-color pump-probe spectra at the LHC-II can be presented. In particular, the presence of non-Markovian effects with respect to the exciton-vibrational interaction in the LHC-II, discovered previously in the two-color pump-probe spectrum, is demonstrated here for the one-color pump-probe case.     

Laser pulse control of exciton dynamics in the FMO complex: Polarization shaping versus effects of structural and energetic disorder

Femtosecond laser pulse control of exciton dynamics in biological chromophore complexes is studied theoretically using the optimal control theory specified to open quantum systems. Based on the laser pulse induced formation of an excitonic wave packet the possibility to localize excitation energy at a certain chromophore within a photosynthetic antenna system (FMO complex of green bacteria) is investigated both for linearly polarized and polarization shaped pulses. Results are presented for an ensemble of N energetically disordered and randomly oriented FMO complexes. Here, the optimized control pulse represents a compromise with respect to the solution of the control task for any individual complex of the ensemble. For the case of an ensemble with N=10 members the polarization shaped control pulse leads to a higher control yield compared with a linearly polarized pulse. This difference becomes considerably smaller for an ensemble with N=120 members. The respective optimized pulses are used to drive excitation energy in a different ensemble with MN complexes to simulate the usual experimental condition in solution. For the case with N=120, the relative control yield coincides with the resulting control yield “in solution”, giving a slightly higher control yield for polarization shaped pulses.     

Synthesis, Characterization, and Reactivity of Group 12 Metal Thiocarboxylates, M(SOCR)(2)Lut(2) [M = Cd, Zn; R = CH(3), C(CH(3))(3); Lut = 3,5-Dimethylpyridine (Lutidine)

A series of group 12 metal thiocarboxylate species, M(SOCR)(2)Lut(2) [M = Cd, Zn; R = CH(3), C(CH(3))(3); Lut = 3,5-dimethylpyridine (lutidine)], were synthesized to investigate their potential to act as precursors for the formation of metal sulfide materials. These species were expected to undergo thiocarboxylic anhydride elimination to give stoichiometric metal sulfides and remove the organic supporting ligands cleanly. These species were characterized by (1)H, (13)C, and (where appropriate) (113)Cd NMR spectroscopies, TGA, elemental analysis, and single-crystal X-ray diffraction. The spectroscopic and analytical data were consistent with the formulas identified above, and in the solid state the compounds are monomeric with approximate tetrahedral metal coordination environments and monodentate S-bond thiocarboxylate ligands. Crystal data for Cd(SOCCH(3))(2)Lut(2): crystallized in the triclinic space group P&onemacr;, with a = 8.267(1) ?, b = 9.467(1) ?, c = 14.087(1) ?, alpha = 94.04(1) degrees, beta = 91.49(1) degrees, gamma = 104.03(1) degrees, and Z = 2. Thermal decomposition of these compounds in the solid state or in solution resulted in formation of the corresponding metal sulfide at low temperatures, as seen by powder X-ray diffraction. Evidence for thiocarboxylic anhydride elimination was documented by NMR in solution phase reactions. The effects on thiocarboxylic anhydride elimination, resulting from varying M, R, or solvent media, were examined by heating NMR tube solutions of M(SOCR)(2)Lut(2) in pyridine or toluene. Heating toluene or pyridine solutions of Cd(SOCCH(3))(2)Lut(2) resulted in formation of nanocrystalline, sphalerite CdS, as determined by X-ray diffraction and TEM. These preliminary reactivity studies have revealed the great potential of this highly tailorable chemical system as precursors to group 12 metal sulfido species.     

Optimal control theory for a target state distributed in time: optimizing the probe-pulse signal of a pump-probe-scheme

Optimal control theory (OCT) is formulated for the case of a two-color pump-probe experiment. The approach allows to calculate the pump-pulse shape in such a way that the probe-pulse absorption signal is maximized. Since the latter quantity is given by the time-averaged expectation value of a time dependent operator (the probe-pulse field-strength times the dipole operator) a version of OCT has to be used where the target state is distributed in time. The method is applied to a molecular three-level system with the pump-pulse driving the transition from the electronic ground state into the first-excited electronic state and the probe-pulse connecting the first-excited state with a higher lying electronic state. Depending on the probe-pulse duration, the vibrational wave packet becomes localized or at least highly concentrated in the Franck-Condon window for the transition into the higher-excited state. The dependence on the probe-pulse duration and on the delay time between the optimized pump-pulse and the probe-pulse is discussed in detail. The whole study demonstrates the feasibility of laser pulse induced temporal wave packet localization and the use of spectroscopic quantities as target states in experiments on femtosecond laser pulse control.     

Hydrogen atom vs electron transfer in catecholase-mimetic oxidations by superoxometal complexes. Deuterium kinetic isotope effects

Dioximato-cobalt(II), -iron(II) and -manganese(II) complexes (1)-(6), acting as functional catecholase and phenoxazinone synthase models, exhibit a deuterium kinetic isotope effect predicted by theory (k4H/k4D < or = 3) in the catalytic oxidative dehydrogenation of 3,5-di-tert-butylcatechol and 2-aminophenol by O2. KIEs in the range of (k4H/k4D approximately 1.79-3.51) are observed with (1) and (2) as catalysts, pointing to hydrogen atom transfer in the rate-determining step from the substrate hydroxy group to the metal-bound superoxo ligand. Less significant KIEs (1.06-1.20) are exhibited by catalysts systems (3)-(6), indicating that proton-coupled electron transfer is the preferred route in those cases.